1. Field of the Invention
An aspect of the invention relates to a switching mode power supply (SMPS) device, an image forming apparatus including the SMPS device, and a method of driving the SMPS device. More particularly, an aspect of the invention relates to a switching mode power supply device capable of preventing electromagnetic interference from occurring due to impedance variance, and reducing power consumption, and an image forming apparatus including the SMPS device, and a method of driving the SMPS device.
2. Description of the Related Art
An image forming apparatus, such as a printer, a photocopier, a facsimile machine, and a multifunctional device capable of combining the functionality of several different pieces of office equipment into a single machine, is a device for printing an image on a print medium by executing a print operation corresponding to an input data.
The image forming apparatus requires a power supply device to convert an AC input voltage into a DC output voltage and supply the DC output voltage to the respective parts such as a print controlling part which has a microcontroller to control printing operation, or a print engine part which accommodates a stack of print media such as printing paper, prints an image onto a print medium, and discharges the print medium with the image formed thereon.
A switching mode power supply (SMPS) device rectifies and smoothes commercial AC into DC, and converts the DC to a high frequency such as 100 kHz, so that an appropriate voltage can be obtained by the transformer.
Methods of controlling an output voltage of an SMPS device generally include a pulse-width modulation (PWM) method of controlling a duty ratio of a switching pulse according to an output voltage variation; a method of controlling a frequency of the switching pulse; and a method of controlling a phase of the switching pulse.
Recently, the functions of the image forming apparatus have been diversified and complicated, and reducing electric power consumption thereof is highly desirable. Accordingly, various methods have been tried to reduce electric power consumption of an SMPS device.
A quasi-resonant control has been applied to an SMPS device as one way of reducing electric power consumption.
The method of quasi-resonant control will be briefly described below.
FIG. 6 is a circuit diagram illustrating a part of an SMPS device performing the quasi-resonant control according to the related art. FIG. 7 is a graph illustrating waveforms in the SMPS device of FIG. 6, and FIG. 8 is a graph explaining the operation of the SMPS device in response to the waveforms illustrated in FIG. 7.
In the quasi-resonant control, the MOS transistor (M_TR) triggers a new cycle by starting a turn-on state when a voltage difference across the M_TR reaches the minimum voltage during switching from the turn-on state to a turn-off state. Accordingly, as the electric power consumption in the M_TR is reduced, the electric power consumption of the SMPS device is also reduced.
Referring to FIGS. 6 through 8, the controller (IC) outputs a switching signal of a predetermined frequency to control switching of the M_TR in response to a voltage supply VCC. Accordingly, a DC input voltage DC_IN that was obtained from an external power supply is rectified and smoothed, and is supplied to the primary winding L11 of the transformer in a predetermined pulse form in accordance with a switching signal as shown in the top half of FIG. 7, which is detected at the point P3 in FIG. 6.
Accordingly, a primitive switching signal having substantially the same frequency as the switching signal is generated at the secondary winding L12 of the transformer, resonated at the resonant frequency which is formed according to the inductance of the secondary winding L12 and the capacitance of the capacitor C11, and input into the controller IC in pulse form as shown in the bottom half of FIG. 7, which is detected at the point P4 in FIG. 6. The primitive switching signal is generated and receives a feedback according to the switching signal of the predetermined frequency which controls switching of the M_TR. The term “primitive switching signal” herein means a signal previously generated in order to output the switching signal by a quasi-resonant control method.
The circuit in FIG. 6 also includes resistors R11, R12, R13, R14, and R15 connected as shown in FIG. 6.
The controller IC drives the SMPS device using quasi-resonant controlling by detecting the voltage of the switching signal having a reference voltage such as 0 voltage, for example, and outputting a switching signal to the M_TR when the time period during which the switching signal below 0 voltage is supplied exceeds a reference time, such as 8 μs, for example.
However, the SMPS device with the above construction has a problem in that the resonance continues until the core is reset according to the inductance and capacitance. Additionally, because the controller IC outputs a switching signal whenever it detects 0 voltage and then voltage under 0 for more than a reference time, current is induced low and switching frequency increases when a light load is driven by the output of the SMPS device.
Electromagnetic interference (EMI) is detected from the harmonics in the low-frequency signal. However, the EMI is detected from the initial frequency component in the high-frequency signal, which causes the increased switching frequency and deterioration of EMI characteristics.
Additionally, as the switching signal is outputted at a shorter interval, turn-on and turn-off operations of M_TR increase with the supply of the light load, compared to when a heavy load is supplied, which subsequently increases power consumption of the M_TR and the SMPS device.